Images of the build-up of missiles on Woody Island in the Paracel Island chain, published by Fox News. Photo: ImageSat International

"Interested parties should work together to maintain peace and stability in the South China Sea region and refrain from taking any unilateral measures that would increase tensions," Taiwanese defence spokesman Major-General David Lo said on Wednesday, declining to give further details.

A satellite image taken by the private company ImageSat International, dated February 14, showed the presence of the equipment, whereas the same area looked to be empty in an image dated February 3.

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The US cable television network Fox News cited a US official as saying the images appeared to show the HQ-9 air defence system, which had a range of about 200 kilometres and could therefore threaten nearby planes.

A US Navy destroyer sailed close to the disputed Paracel Island chain, which includes Woody Island, in a "freedom of navigation" exercise late last month. China branded that action "highly dangerous and irresponsible" and accused the US of being "the biggest cause of militarisation in the South China Sea".

China, Taiwan and Vietnam have competing claims in the area and the US has objected to any "militarising" of the islands.

The Diplomat also reported last week that China's South China Sea island-building had expanded into the Paracels, which are also claimed by Vietnam and Taiwan.

The report has not been confirmed.

Images that were separately obtained appeared to show dredging and filling at two new sites in the Chinese-held island chain and the construction of a helicopter base.

China has already constructed airstrips and naval berths capable of use for military purposes on the islands.

Confirmation of the deployment of missile launchers came as the US and ASEAN issued a joint statement at the California summit that called for "maritime security" but did not specifically mention the South China Sea, indicating division among regional countries on how to counter China's assertiveness.

Countries like the Philippines and Vietnam favour a more aggressive approach while other nations like Cambodia and Laos, which have closer ties with Beijing, are reluctant to directly challenge China's behaviour.

China late last year provoked condemnation when it landed civilian planes on an artificial island where it has built infrastructure that can accommodate military aircraft.

Vietnam has accused China of towing a $1 billion oil rig into disputed waters in a potential rerun of a stand-off that sparked violent anti-Chinese riots in Vietnam in 2014.

The United States has obtained final approval to expand its military presence in the Philippines and has begun making spy flights over the region in Boeing P-8A Poseidon aircraft based in Singapore.

Carlyle Thayer, an expert on the South China Sea from Australia's Defence Force Academy, said the deployment of the HQ-9 missile system raises the stakes for future US maritime patrols.

The system is capable of threatening carrier-based planes coming to the assistance of any US Navy warship confronted by China during freedom of navigation exercises, he said.

Professor Thayer said the deployment of such a sophisticated and lethal air defence system was no doubt in response to US aerial activities and the patrol near Triton island. The deployment was also a demonstration that at short notice China can deploy similar systems to other disputed islands on the pretext of a threat from the United States.

Professor Thayer said that until now concerns of the US and other countries had focused on artificial island development by China.

But China first constructed an airstrip on Woody Island in 1990 than can accommodate fighter jets.

Existing facilities on the island include naval docks, precision approach radar, a fuel depot and military facilities, including troop accommodation.

Rory Medcalf, head of the National Security College at the Australian National University, said the reports of China placing surface-to-air missile batteries on Woody Islands were "sadly not surprising".

"This will further militarise the tensions in the South China Sea. It reinforces the view that China intends to exert growing control in these international waters, including potentially by declaring an air defence identification zone.

"It is also against the spirit, if not the letter, of Xi Jinping's assurance last year that China was not putting weapons on the artificial islands it has made."

Professor Medcalf said the move showed China did not take diplomatic efforts on the disputed territories seriously.

"Technically, Woody Island is a real island, not an artificial one, but China's possession of it is disputed by Vietnam and Taiwan.

"China seems to be putting missiles on a disputed South China Sea island while going slow on negotiating a code of conduct with ASEAN that would ban precisely this kind of thing. This is a sign that China does not take such diplomacy seriously."